Some display standards require the display of data in a compressed format. For example the PictBridge standard from the Camera & Imaging Products Association (CIPA) allows images to be printed directly from digital cameras to a printer without having to connect the camera to a computer. Such printers currently must conform to at least the Exif/JPEG standard so that products from various vendors are compatible with each other. In the test and measurement field, it is often desired to print out the image that appears on an instrument screen, wherein the screen image represents acquired data. Therefore, it is desirable that the output which is to be printed from the instrument be in a format that is acceptable by printers which conform to the PictBridge standard. That is, the image data should be in the Exif/JPEG format, which requires either 4:2:2 or 4:2:0 chrominance downsampling.
When the original data comes from a low resolution source, such as an oscilloscope screen (typically 320×240, a “quarter VGA” display), reconstruction of the data from the lossy compressed format may cause color distortion or smearing, leading to an improper interpretation of the original data. Using JPEG as an example, the data from the low resolution source is in the form of pixels, each pixel having a luminance value (Y) and a pair of chrominance values (U,V). Where all three values are given for each pixel, the format is referred to as being in a 4:4:4 format. However it is often desired to compress these pixels for transmission or storage, and then later decompress them for display. The Exif2.1 JPEG format specifies that either 4:2:2 or 4:2:0 chrominance downsampling be used for storage and transmission of the original JPEG image data. Chrominance downsampling is specified because the human eye is less sensitive to chrominance variations than to luminance variations.
An example of chrominance downsampling is the following conversion of the first four pixels of a display line:
{4:4:4} [Y0U0V0][Y1U1V1][Y2U2V2][Y3U3V3]
{4:2:2} [Y0U0] [Y1V1] [Y2U2] [Y3V3]
When a printer, for example, converts the 4:2:2 bit stream back into viewable pixels having luminance and both chrominance components {4:4:4}, the following pixels are produced:
[Y0U0V1][Y1U0V1][Y2U2V3][Y3U2V3]
As is apparent, the chrominance components of the decompressed pixels are not identical to those of the original pixels, i.e., U0V0≠U0V1 for the first pixel. This discrepancy occurs because neighboring pairs of compressed pixels are used to reconstruct the decompressed pixels. Within each pair of reconstructed pixels [Y0U0V1][Y1U0V1] or [Y2U2V3][Y3U2V3], is actually sharing a chrominance value from its neighboring pixel. That is, first reconstructed pixel (with luminance value Y0) is using the V1 chrominance value copied from its neighbor on the right, while the second reconstructed pixel (with luminance value Y1) is using the U0 chrominance value copied from its neighbor on the left. This reconstructed pattern is repeated for each pair of adjacent pixels.
Referring to FIG. 1a, a low resolution pixel row is shown with original data that has a solid color transition between pixels 2 and 3. In a low resolution rendering, when recreated after JPEG compression and decompression, this may show up as color smearing or color artifacts, as shown in FIG. 1b, since the original pixels have been approximated for the reconstruction due to the lossy nature of the JPEG compression. In particular pixel pairs 2/3 and 12/13, when recreated, show color artifacts along edges of regions of solid color, where the color artifacts may be of a completely different color than in the original pixel colors. Such artifacts occur when the color transition occurs between pixels in a pixel pair.
What is desired is an apparatus and method of formatting the compressed data such that reconstructing the original low resolution picture from a lossy compressed version occurs without chrominance compression induced color smearing or color artifacts.